[LTP] [PATCH] syscalls/ioctl_loop05: Get the logic_block_size dynamically

Yang Xu xuyang2018.jy@cn.fujitsu.com
Tue Jun 9 12:46:17 CEST 2020

Hi Jan

> ----- Original Message -----
>> Hi Jan
>>> ----- Original Message -----
>>>> In loop driver code, the sb_bsize was calculated as below
>>>> sb_bsize = bdev_logical_block_size(inode->i_sb->s_bdev),
>>>> it is the super block's block size that the backing file's inode belongs
>>>> to,
>>>> not by using the st_blksize member of stat struct(it uses
>>>> inode->i_blkbits).
>>> I'm not sure I follow the above, are you saying the difference is bdev
>>> blksize
>>> vs. filesystem blksize?
>> I said the loop driver used  dev_logical_block_size(inode->i_sb->s_bdev) but
>> not using
>> st_blksize.
> I know, but I'm trying to understand what the difference is between those two.
AFAIK, st_blksize is used to standard io in user space as man-page said 
"/* Block size for filesystem I/O */", it maybe a buffer write used. It 
is a  block size that block in inode used.
But dev_logical_block_size is a min unit thatrequest queue used on block 

ps: this is my understanding. If it's wrong, please correct me.
>>> Would BLKSSZGET work? It returns bdev_logical_block_size().
>> But it needs a blockdev, in user space, we can specify bdev, but how can we
>> figure out this inode->i_sb->s_bdev block dev.
> Isn't that the block device "test.img" is on?
Do you mean the test.img belong to some block dev, such as /dev/sda1 or 
our mounted block_dev? If so, I think it is.

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